Description and evaluation of the Atmospheric Radionuclide Transport Model ARTM

Kurzfassung

Despite of filtering techniques, nuclear facilities emit small amounts of radioactive tracers to the air. Detailed measuring of the effects of such discharges to the environment in the vicinity of the emitter covering an area of several square kilometres is very challenging. Fast and efficient atmospheric dispersion models are thus required to monitor and predict atmospheric dispersion of trace species in the vicinity of nuclear facilities. In Germany, the Lagrangian Atmospheric Radionuclide Transport Model (ARTM) was designed for this purpose. It simulates the dispersion of discharges from nuclear facilities typically up to 20 km distance within the planetary boundary layer. Such transport models have to be validated carefully to make sure that they simulate tracer dispersions comparable to reality. This study shows the description and extension of ARTM as well as the analysis of the three-dimensional dispersion properties and their evaluation. In a sensitivity study, the effects of stability class, roughness length, zero-plane displacement, source height and tracer type on the three-dimensional plume dispersion are analysed. Furthermore, the dispersion of five turbulence models, the default turbulence models of ARTM 2.8.0 and ARTM 3.0.0, one alternative built-in turbulence model of ARTM and two further turbulence models newly implemented into ARTM are studied. Airborne CO2 observations in the vicinity of the lignite power plant Be lchat´ow, Poland, during the CoMet campaign in 2018 allow to evaluate the model performance under unstable boundary layer conditions. An intercomparison of ARTM with numerical weather prediction and large-eddy simulation models extend the investigation to slightly unstable atmospheric conditions. The results show that the stability class, as a parametrisation of the atmospheric stability, causes the largest impact and hence the largest uncertainty in the simulation results. The usage of measured Obukhov lengths substantially improves the accuracy because of its continuous stability parametrisation. The turbulence model of ARTM 3.0.0 show the largest deviation from the well-mixed state by up to approx. 20%. All simulated mixing ratios are in the same order of magnitude as the airborne in situ data. The turbulence setups of ARTM 2.8.0 and 3.0.0 underestimate the plume widths by up to 50%. The three other turbulence models agree better with the observations simulating comparable plume widths. The intercomparison reveals that, in contrast to very unstable boundary layer conditions, the turbulence model of ARTM 2.8.0 delivers comparable results to those of the other transport models under slightly unstable conditions. The results of this work may help to improve the accuracy of plumes simulated by ARTM representing real plumes in very unstable atmospheric conditions by the selection of distinct turbulence models.